Field of the Invention
The present invention relates to a fixing device, and more specifically to an external heating type fixing device including an infrared heater.
Description of the Related Art
In a fixing device used in an electrophotographic image forming device, in an internal heating method where an infrared heater is arranged inside a fixing roller, an inner circumferential surface of the fixing roller is heated first and the heat is gradually transferred to a surface of the fixing roller, so that the temperature of the surface of the fixing roller rises slowly and a warm-up time tends to be long. On the other hand, in an external heating method where an infrared heater is arranged outside a fixing roller, a surface of the fixing roller is directly heated, so that the temperature of the surface of the fixing roller rises fast and the warm-up time is reduced.
Here, when a so-called thermal runaway occurs in which the infrared heater is continuously turned on, an entire external circumferential surface of the fixing roller is equally heated in the internal heating method, so that when a temperature detected by a temperature sensor arranged on the surface of the fixing roller exceeds a predetermined temperature, a power supply to the infrared heater is shut down and the fixing roller is prevented from fuming and firing. On the other hand, in the external heating method, in a state in which the fixing roller is stopped, only an area where infrared rays are irradiated from the infrared heater is intensively heated and a temperature detection area where infrared rays are not irradiated is not heated directly, so that in a configuration in which the temperature of the surface of the fixing roller is detected, there is a risk that the power supply to the infrared heater is not shut down even when the thermal runaway occurs.
JP 2002-72757 A discloses an external heating type fixing device which includes a heat conducting member that receives heat from a heating source and transfers the heat to a fixing roller by being in contact with the fixing roller, a temperature detecting means that detects a temperature of the heat conducting member, and a temperature control means that controls a heating temperature of the heating source, and in which the temperature control means shuts down power supply to the heating source when the temperature detecting means detects a temperature higher than a predetermined setting temperature.
In the fixing device disclosed in JP 2002-72757 A, it is possible to quickly shut down the power supply to the heating source against the thermal runaway of the heating source when the fixing roller is stopped. However, regarding the thermal runaway of the heating source when the fixing roller is rotated, the temperature at a contact portion between the heat conducting member and the fixing roller is hard to rise, so that there is a risk that it takes time for the power supply to the heating source to be shut down or the power supply is not shut down.
Further, in the fixing device disclosed in JP 2002-72757 A, there is a risk that a temperature detection sensitivity is degraded even when the contact portion of the heat conducting member is slightly separated from the fixing roller because of, for example, deformation of the contact portion of the heat conducting member during paper jam.